Optical waveguides generally comprise an optical guiding region of a refractive index n.sub.1, embedded in material of a refractive index n.sub.2 where usually n.sub.2 &lt;n.sub.1. It should be observed that the guiding region as well as the bedding material may themselves be structured by having two or more regions of different refractive indices, as illustrated by numerous known designs of optical fibres.
It is well known that optical fibres of the kind used in optical communications and for optical fibre sensors, for example, are usually covered with a protective coating to protect the fibre surface from mechanical and chemical damage.
It is known also to reduce temperature dependence of transmission losses of an optical fibre by application to the fibre of an appropriate coating. For example, published European patent application EP-A-0076575 ("Optical fibre insensitive to temperature variations", Hughes Aircraft Company) discloses an optical fibre suitable for operation at high temperatures. According to the disclosure in EP-A-0076575 the temperature dependence of transmission losses, i.e. attenuation of optical signals passing through the fibre, at high operating temperatures is reduced by applying a metal coating to the fibre and annealing. The coating, which may be aluminium or another metal or a metal alloy, is applied to the fibre by pulling the fibre through a melt bath, for example, and the fibre so coated is then annealed at a temperature of several hundred degrees Celsius. It is stated in EP-A-0076575 that, provided the annealing temperature is sufficiently high for the transmission losses through the fibre to be much the same as those at room temperature, the temperature dependence of the transmission loss is substantially eliminated over a range of temperatures from -200.degree. C. to 560.degree. C.
The effect of temperature variations on the transmission loss of optical fibres is considered also in "Optimum Design of Coated Optical Fibres Considering Excess at Low Temperature", K Masuno and K Ishihara, J Opt. Comm., 3(1982) 4, pp 142-145. The effect of temperature variations on transmission loss is considered with reference to optical fibres coated with a nylon coating, and it is suggested that temperature dependence of transmission loss at low temperatures can be kept very low by employing nylon coatings with a linear thermal coefficient of expansion of the order of 10.sup.-5 .degree. C..sup.-1.
It is an object of the present invention to reduce temperature dependence of transmission delay in optical waveguides.
According to one aspect of the present invention a method of reducing temperature dependence of transmission delay in an optical waveguide comprises attaching the waveguide to stressing means arranged to apply to the optical waveguide a temperature dependent stress such that changes in transmission delay induced by the applied stress counteract the temperature induced delay changes.
According to another aspect of the present invention an optical waveguide assembly comprises an optical waveguide and attached to the optical waveguide stressing means capable of applying temperature dependent stress to the attached waveguide such that changes in transmission delays induced by the applied stress counteract temperature induced delay changes therein.
The optical waveguide is conveniently an optical fibre. Alternatively, the optical waveguide may, for example, comprise an optical waveguide structure in which the guiding region is embedded in a planar substrate, such as for example, a LiNbO.sub.3 (lithium niobate) thin film waveguide structure.
The stressing means may be attached to the waveguide at discrete spaced positions, or may be in intimate contact with the waveguide or with an intermediate material itself attached to the waveguide.
The attachment between waveguide and stressing means may be solely by way of friction, or may be by means of, for example, an adhesive compound.
Preferably the stressing means comprises a sleeve about the waveguide.
Alternatively the stressing means may, for example, be a support member such as, for example, a strength member of an optical fibre cable.
In a preferred embodiment of the present invention the optical waveguide comprises an optical fibre, and the stressing means comprise a sleeve forming a jacket tightly fitting around the optical fibre or at least part of the length thereof.
Temperature-induced changes in transmission delay in for example, an optical fibre are caused by a combination of changes in the length of the fibre and in the refractive index of the fibre. These changes in transmission delay are counteracted in accordance with the present invention by strain in, and/or changes in the refractive index of, the fibre which result from the applied stress. The jacket may be chosen such that the changes in transmission delays caused by the action of the jacket counteract the thermally induced changes to such a degree as to substantially compensate therefor.
In a further preferred form of the present invention the optical fibre is of a material composition and structure such that the coefficient of linear thermal expansion of the fibre determines the overall change in the transmission delay, and the jacket comprises material having a coefficient of linear thermal expansion opposite to that of the fibre.
In a yet further preferred form, the present invention comprises an optical fibre having a positive coefficient of linear thermal expansion, and a jacket of a material having a negative coefficient of linear thermal expansion.
The jacket may conveniently be formed of a liquid crystalline polymer. The polymer may be extruded onto the optical fibre.
The present invention may be employed, for example, to provide a substantially temperature independent optical path length reference.
Thus, optical fibres are known to be useful, for example, as interferometric sensor elements owing to their inherent sensitivity to changes in temperature, strain, pressure and electric current and magnetic field. However most of these measurements in sensing require the ability to distinguish between the parameter being sensed and the other influences which may have a similar effect on the sensing properties of the fibre. This is often achieved by using a reference fibre which is subjected to the same influences as the sensing fibre, except for the one parameter to be measured. This requires careful layout in the design of the sensor. Some control element is also included in the reference arm to keep track of the drift induced by differential effects, especially when temperature is a noise source.
The present invention overcomes or at least mitigates some of these problems by providing inter alia an optical fibre which is coated with a material which has the effect of de-sensitizing the optical delay in the fibre with respect to changes in temperature.